This is a note to let you know that I've just added the patch titled
ring-buffer: Fix wrong stat of cpu_buffer->read
to the 5.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
ring-buffer-fix-wrong-stat-of-cpu_buffer-read.patch
and it can be found in the queue-5.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit f405630a7f0d80304dbf43eaaec1a4fdc84a0a7a
Author: Zheng Yejian <zhengyejian1@xxxxxxxxxx>
Date: Mon Jul 24 13:40:40 2023 +0800
ring-buffer: Fix wrong stat of cpu_buffer->read
[ Upstream commit 2d093282b0d4357373497f65db6a05eb0c28b7c8 ]
When pages are removed in rb_remove_pages(), 'cpu_buffer->read' is set
to 0 in order to make sure any read iterators reset themselves. However,
this will mess 'entries' stating, see following steps:
# cd /sys/kernel/tracing/
# 1. Enlarge ring buffer prepare for later reducing:
# echo 20 > per_cpu/cpu0/buffer_size_kb
# 2. Write a log into ring buffer of cpu0:
# taskset -c 0 echo "hello1" > trace_marker
# 3. Read the log:
# cat per_cpu/cpu0/trace_pipe
<...>-332 [000] ..... 62.406844: tracing_mark_write: hello1
# 4. Stop reading and see the stats, now 0 entries, and 1 event readed:
# cat per_cpu/cpu0/stats
entries: 0
[...]
read events: 1
# 5. Reduce the ring buffer
# echo 7 > per_cpu/cpu0/buffer_size_kb
# 6. Now entries became unexpected 1 because actually no entries!!!
# cat per_cpu/cpu0/stats
entries: 1
[...]
read events: 0
To fix it, introduce 'page_removed' field to count total removed pages
since last reset, then use it to let read iterators reset themselves
instead of changing the 'read' pointer.
Link: https://lore.kernel.org/linux-trace-kernel/20230724054040.3489499-1-zhengyejian1@xxxxxxxxxx
Cc: <mhiramat@xxxxxxxxxx>
Cc: <vnagarnaik@xxxxxxxxxx>
Fixes: 83f40318dab0 ("ring-buffer: Make removal of ring buffer pages atomic")
Signed-off-by: Zheng Yejian <zhengyejian1@xxxxxxxxxx>
Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index afd7f3a51485e..445475c229b3a 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -485,6 +485,8 @@ struct ring_buffer_per_cpu {
unsigned long read_bytes;
u64 write_stamp;
u64 read_stamp;
+ /* pages removed since last reset */
+ unsigned long pages_removed;
/* ring buffer pages to update, > 0 to add, < 0 to remove */
long nr_pages_to_update;
struct list_head new_pages; /* new pages to add */
@@ -520,6 +522,7 @@ struct ring_buffer_iter {
struct buffer_page *head_page;
struct buffer_page *cache_reader_page;
unsigned long cache_read;
+ unsigned long cache_pages_removed;
u64 read_stamp;
};
@@ -1581,6 +1584,8 @@ rb_remove_pages(struct ring_buffer_per_cpu *cpu_buffer, unsigned long nr_pages)
to_remove = rb_list_head(to_remove)->next;
head_bit |= (unsigned long)to_remove & RB_PAGE_HEAD;
}
+ /* Read iterators need to reset themselves when some pages removed */
+ cpu_buffer->pages_removed += nr_removed;
next_page = rb_list_head(to_remove)->next;
@@ -1602,12 +1607,6 @@ rb_remove_pages(struct ring_buffer_per_cpu *cpu_buffer, unsigned long nr_pages)
cpu_buffer->head_page = list_entry(next_page,
struct buffer_page, list);
- /*
- * change read pointer to make sure any read iterators reset
- * themselves
- */
- cpu_buffer->read = 0;
-
/* pages are removed, resume tracing and then free the pages */
atomic_dec(&cpu_buffer->record_disabled);
raw_spin_unlock_irq(&cpu_buffer->reader_lock);
@@ -3659,6 +3658,7 @@ static void rb_iter_reset(struct ring_buffer_iter *iter)
iter->cache_reader_page = iter->head_page;
iter->cache_read = cpu_buffer->read;
+ iter->cache_pages_removed = cpu_buffer->pages_removed;
if (iter->head)
iter->read_stamp = cpu_buffer->read_stamp;
@@ -4101,12 +4101,13 @@ rb_iter_peek(struct ring_buffer_iter *iter, u64 *ts)
buffer = cpu_buffer->buffer;
/*
- * Check if someone performed a consuming read to
- * the buffer. A consuming read invalidates the iterator
- * and we need to reset the iterator in this case.
+ * Check if someone performed a consuming read to the buffer
+ * or removed some pages from the buffer. In these cases,
+ * iterator was invalidated and we need to reset it.
*/
if (unlikely(iter->cache_read != cpu_buffer->read ||
- iter->cache_reader_page != cpu_buffer->reader_page))
+ iter->cache_reader_page != cpu_buffer->reader_page ||
+ iter->cache_pages_removed != cpu_buffer->pages_removed))
rb_iter_reset(iter);
again:
@@ -4538,6 +4539,7 @@ rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
cpu_buffer->last_overrun = 0;
rb_head_page_activate(cpu_buffer);
+ cpu_buffer->pages_removed = 0;
}
/**
